The market for electric bicycles (e-bikes), particularly those with lithium-ion (Li-ion) batteries, is growing worldwide. When designing power systems for e-bike controllers, it’s important to pay close attention to overall power conversion efficiency and total solution size. This paper discusses ways to implement small, efficient power subsystems for e-bikes.
Introduction: Meeting Power Subsystem Demands for a Growing Market
With nearly 35 million unit sales worldwide1, the global e-bike market is huge.
According to Statista, in 2023, global sales of e-bikes are forecast to reach approximately 40 million units.
China has the lion’s share of the e-bike market, but most of these bikes are based on older sealed lead acid (SLA) battery technology. There are two important market trends in the China e-bike market:
- First the China e-bike market is seeing a transition from SLA batteries to new and lighter Li-ion battery chemistry – which requires more complex electronics
- Second, we are seeing higher battery capacity driving higher voltages (up to 54V nominal) to increase the riding range of the e-bikes
Moving to a higher voltage Li-ion chemistry involves developing a control unit which is small and light but still provides enough range to make the bike usable in an urban environment. This means significant system design challenges, especially as it relates to the power subsystem.
System Specifications for E-Bike Controllers
A generic system block diagram of the e-bike controller is shown in Figure 1.
The power for the system comes from the Li-ion battery, which can be 36V and up, depending on the number of battery cells used. This Li-ion battery powers the motor, the MOSFET drivers, the microcontroller as well as other ancillary things like the horn, lights, and hall-effect sensor in the main motor (mostly a brushless DC motor). A generic power architecture is shown in Figure 2.
Existing e-bike controllers are large and bulky; the new controllers are designed to fit within the frame and/or under the seat. This poses two new requirements for the Li-ion battery controller: small solution size and very low heat generation, which is now being dissipated in a much smaller area.
Power Solution Efficiency Impacts Heat Dissipated and E-Bike Range
The range of an e-bike depends on how large the battery pack is, but it is also impacted by the power conversion efficiency of the power subsystem used.
Let’s consider an e-bike with a 36V/10Ah battery pack. The energy in the battery pack is 36V x 10Ah = 360Wh (Watt.Hour). The usable energy in the battery depends on the discharge rate, the ambient temperature, etc. For simplicity, let’s assume all this energy is available to drive the bike. Now, assume the energy used per mile is 14.4Wh (a very typical number for e-bikes). Here is what happens as the power system efficiency goes up:
At 80% efficiency:
Power used for driving the motor = 360Wh x 80% = 288Wh
Range of this e-bike = 288Wh / (14.4Wh/mi) = 20 miles
Power dissipated as heat = 360Wh – 288Wh = 72Wh (or 259kJoules)
At 90% efficiency:
Power used for driving the motor = 360Wh x 90% = 324Wh
Range of this e-bike = 324Wh / (14.4Wh/mi) = 22.5 miles
Power dissipated as heat = 360Wh – 324Wh = 36Wh (or 130kJoules)
Moving power system efficiency from 80% to 90% can give you a 12.5% increase in range and 50% reduction in heat generation that must be dissipated. Li-ion battery & electronics life degrades with temperature making this a critical decision making criteria.
Table 1 below summarizes the range and heat generated with different power conversion efficiencies.
Let’s consider some of the latest synchronous DC-DC converters in the market, and see how their specifications can enable an optimum power subsystem for an e-bike controller.
Popular Power ICs for e-bike
Let’s go through some power design examples using a couple of popular parts in the market today. Design requirement:
Vin range: 27VDC to 42VDC
Vout: 5V @2A
Let’s start with Maxim’s MAX17503, a popular 4.5V-60V, 2.5A, high-efficiency, synchronous step-down DC-DC converter. Using the EE-Sim DC-DC Converter tool and choosing a balance design between efficiency and size, the complete power system looks as shown:
With a relatively small inductor (10uH) this power solution provides an 86.5% power conversion efficiency – Vin = 36V, Vout = 5V @ 2A running at 470kHz switching frequency. The total solution footprint for this power subsystem is 156 mm2, as shown in Table 2 below. The external inductor specifications and the small package of the IC itself help with the ultra-small solution size.